The approach to fabricate “Nanoscale Environmental Barrier Materials” by employing precisely-coated nanophase particles that have been consolidated to near theoretical density offers unprecedented opportunities in fabricating bulk quantities of materials that currently exist only as small research samples (because of prohibitively expensive fabrication techniques. Since the properties of these materials are controlled to a great extent by interfaces, these materials are called “Interface-Controlled Nano-Materials.”
There are many, recent uses of nanophase particles in which either the reduction in size of particles to the nanometer scale (typically <100 nm) or dispersement of the particles as a second solid phase can be used to produce materials with unique properties. Nanophase particles consolidated into monolithic materials exhibit greater hardness and strength in metals and cermets due to reduced grain size and slip distance, respectively. Nanosized grains in metals provide high ductility while enhancing other properties, which is not possible with normal grain-size metals. In ceramics, greater hardness and toughness result from reduced defect size and enhanced grain boundary stress relaxation, even at ambient temperature. With nanosize grains diffusivity can be greatly increased due to the larger volume of grain boundaries, while thermal conductivity is reduced because of enhanced phonon scattering at grain boundaries and other nanoscale features.
The approach of creating highly desirable properties in materials by developing specific micro- and nano-structures is not new. In fact it has been used for thousands of years. Generally speaking, techniques of materials processing (such as precipitation hardening of aluminum and copper alloys, carbonization and heat treatment of steel, inducing color to glass, and making porcelain) are all based on developing unique micro- and nano-structures in the materials. In fact, it can be said that the whole field of Materials Science is exactly that—searching for the right nano- and microstructure and the methods of arriving at it to yield the desired properties.
The approach utilizing nanosize particles to yield bulk materials, or coatings, is relatively new. Consolidation of fine powders constituted of nanosize particles is becoming a well-established industrial technology. There are numerous methods and techniques for fabricating nanosize particles, protecting and modifying their surfaces, and consolidating them via colloidal methods, cold pressing, sintering, or hot pressing. In some applications, despite their increased cost, the use of nanosize particles brings about such unique and desirable properties to the produced materials that whole new industries have been established. One of the main advantages of using nanosize particle based powders as starting materials versus using conventional microscopic-based powders lies in the remarkable reactivity of nanosize particles.
Nanosize particles have also been coated with a nanoscale coating of a different composition in numerous existing applications. In all these applications, the vast majority of which are medical, the particles maintain nano-dimensions and remain discrete. Metallic or ceramic nanosize particles, for example, can be coated with biochemicals, such as proteins and peptides for binding to and destroying particular cells with medicine, heat, or by cutting off blood supply. Many of these nanosize particles are paramagnetic so that they can be imaged with increased contrast by MRI and can be targeted to a desired location by a magnetic field that is external to the body. These paramagnetic nanosize particles are first coated with gold or a bio-compatible polymer and then coated with a medicine or vaccine. A magnetic field is employed to direct these nanosize particles to the area of disease so that the medicine coating may be released to the precise location where it is needed. This allows the use of very effective drugs that are otherwise too toxic to be delivered. Alternatively, this site specific feature can also be used to direct hyper-thermal treatment to a tumor. The radio frequency energy source for this thermal therapeutic treatment is external to the body. Additionally, layered composite coatings of both biodegradable and bioactive compounds provide the medical community with great flexibility and precise control over drug design and release. When nanosize particles of bio-active crystals or materials are coated with one or more polymer layers, they can be tailored for slow, quick, specific, or heat-release of drugs.
Other applications of nanoscale coatings of discrete nanoparticles include coatings to keep nanoparticles from fusing together at contact, to inhibit particle growth (see, e.g., U.S. Pat. No. 6,048,577), to inhibit oxidation of energetic materials, to inhibit photocatalytic reaction of the particle, and coatings to alter the optical properties of the nanoparticles, such as photoluminescence (see, e.g., U.S. Pat. No. 6,861,155). It is interesting to note that in all of these existing applications of coated nanoparticles, which also includes cosmetic and sunscreen additives, the particles remain discrete and the composition of the particle is inorganic, while the composition of the exterior coating is organic.
Utilizing micron-sized particles, the Vapor Phase Redistribution (VPR) may be used for fabricating novel ceramic/metal composites from ceramic/metal systems that are not characterized by the favorable interfacial wetting phenomena, such as that utilized so successfully in the manufacturing of WC/Co materials (see, e.g., U.S. Pat. No. 4,943,320). The VPR techniques consist of intimately mixing metal and ceramic powders under a controlled atmosphere and placing the resulting mixture under vacuum or a controlled atmosphere at elevated temperatures to allow for the metal phase to deposit on the surface of ceramic particles via a vapor deposition process. As a result of this procedure, most of the ceramic particles become coated on the surface with a thin layer of metal. Afterwards, the resultant mixture is rapidly compressed to facilitate binding of the metal-capped particles and to produce a dense consolidated part. Although the VPR procedure shares some similarities in its general approach to the instant invention, there are deep and important differences between the two approaches that make the instant approach unique and different. For example, and in contrast to the current teaching, which requires totally and evenly coated nanoscale particles for its success, the VPR approach is not designed to work with nanosize particles and does not yield good uniform coating for each particle.